Previously we demonstrated that administration of synthetic progesterone to macaques resulted in thinning of the vaginal epithelium and increased susceptibility to SIV infection following atraumatic vaginal inoculation. The objective of this study was to determine whether administration of progestin-only contraceptives also induced thinning of the vaginal epithelium in macaques. Eight intact rhesus monkeys (4/group) were treated with either a single intramuscular injection of 30 mg Depo-Provera or subcutaneous insertion of Norplant-II [unreadable] (2x75 mg rods; day 0). Norplant-II rods were removed 90 days after insertion. Vaginal biopsies were obtained during a pretreatment menstrual cycle and following treatment on days 10, 30, 60, 118, and 146. Formalin-fixed vaginal biopsies were evaluated for epithelial thickness and the degree of keratinization. Circulating levels of estradiol and progesterone confirmed the stage of the menstrual cycle in which pretreatment biopsies were obtained. A pronounced effect of progestin treatment was observed on the vaginal epithelium. Compared to pretreatment menstrual cycle controls, the vaginal epithelial thickness was decreased (p<0.05) by day 30 or 60 following Norplant-II insertion or Depo-Provera injection, respectively. The number of epithelial cell layers was also decreased (p<0.05) on days 30 and/or 60 in progestin-treated monkeys compared to pretreatment control cycles. Following removal of Norplant-II or metabolic excretion of MPA, the vaginal epithelium regenerated and the thickness was no longer different (p>0.05) from the pretreatment control cycle These data demonstrate that progestin-only contraceptives induced thinning of the vaginal epithelium in rhesus monkeys, and this effect was rapidly reversible following physical or metabolic removal of the progestin.